340 Dr. Sharpey on the Mechanism of Respiration 



powder which remains outside the gill is carried along its surface 

 ia straight lines from the base to the margiOj along which it then 

 advances onwards towards the fore part of the animal. As the 

 spaces between the layers of all the gills terminate directly or indi- 

 rectly at the excretory orifice, it is easily conceivable that the wa- 

 ter, penetrating by the entire surface of these organs, may, by their 

 concentrated effect, give rise to the powerful current which is ob- 

 served to come out from the animal. 



On examining a portion of the gills with a powerful lens, I per- 

 ceived that it was beset with minute cilia, which are evidently in- 

 strumental in producing the different currents. Most of them are 

 ranged along the anterior and posterior margin of each of the vessels 

 composing the giUs, in two sets, one nearer the surface, consisting 

 of longer and more opaque cilia ; the other close to the first, but a 

 little deeper, in which they are shorter and nearly transparent. 

 Both sets are in constant motion, but of this it is difficult to con- 

 vey a correct idea by description. The more opaque cilia, or those 

 of the exterior range, appear and disappear by turns, as if they 

 either M^ere alternately pushed out and retracted, or were continu- 

 ally changing from a horizontal to a vertical direction. The mo- 

 tion of the other set appears to consist in a succession of undula- 

 tions, which proceed in a uniform manner along the margin of the 

 vessel from one end to the other. It resembles a good deal the 

 apparent progression of the turns of a spiral when it revolves on 

 its axis, and might very easily be mistaken for the circulation of a 

 fluid in the interior of a canal, more particularly as the course of 

 the undulations is different on the two edges of the vessel, being 

 directed on the one towards the margin of the gill, and on the other 

 towards the base. But besides that the undulations continue to go 

 on for some time in small pieces cut off from the gill, which is in- 

 consistent with the progression of a fluid in a canal, the cilia are 

 easily distinguished when the undulatory motion has become lan- 

 guid. When it has entirely stopped they remain in contact with 

 each other, so as to present the appearance of a membrane attach- 

 ed to the edge of the vessel. 



It is very remarkable, that when the gill is immersed in fresh 

 water, both the currents and the motion of the cilia are almost in- 

 stantaneously stopped. 



Such, nearly, are the appearances I have observed in the mussel. 

 There are, indeed, other particulars which could not well be made 

 intelligible in a short description, and which I purposely avoid 

 mentioning here, the object of this paper being merely to give a 

 statement of the phenomena in general, and not to detail the pecu- 

 liarities of individual cases, which I shall be able to do with more 

 advantage on a future occasion. For this reason, also, I shall con- 

 tent myself with a simple mention of the other animals belonging 

 to this class, in which I have observed the currents. Of the Bi- - 

 valves, I have examined the fresh water mussel, the oyster, arid 

 another species. In the first of these the motion of the two sets of 



